<jats:title>Abstract</jats:title> <jats:p>The North Pacific Subtropical Fronts (STFs), accompanied by the eastward-flowing subtropical countercurrent, stretch from the western Pacific Ocean to the north of Hawaii. Previous work has detected different trends of the frontal position and strength between the western STF (WSTF; west of 180°) and the eastern STF (ESTF; east of 180°) in the past 40 years. However, whether the basin-scale STFs have zonally asymmetric variability on multidecadal time scales and what drives that change remain to be quantified. Our recent work has shown that the multidecadal variability of the WSTF is controlled by the Atlantic multidecadal oscillation via the subtropical mode water variability. The present study proposes that the variability of ESTF is modulated by the Pacific decadal oscillation (PDO) via the central mode water (CMW) variability, quasi synchronously on multidecadal time scales. During a PDO positive phase, the enhanced midlatitude westerly winds in the central North Pacific increase the local surface buoyancy loss and deepen the winter mixed layer, which enlarges the CMW formation and thus increases its volume. Meanwhile, accompanied by the southward-migrated outcropping zone, the main body of CMW shifts equatorward. In response to such CMW changes, the ESTF strengthens and shifts equatorward correspondingly. Conversely, during a PDO negative phase, the weakened midlatitude westerly winds in the central North Pacific decrease the local surface buoyancy loss and shallow the winter mixed layer, which reduces the CMW formation and thus decreases its volume. Meanwhile, accompanied by northward-migrated outcropping zone, the main body of CMW shifts poleward. In response to such CMW changes, the ESTF weakens and shifts poleward correspondingly. Our results reveal that the dominant factor controlling the low-frequency variability of the WSTF and ESTF is different, which renews the conventional picture that all of the STFs behave symmetrically, with important implications for the North Pacific climate variability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent evidence shows that the North Pacific subtropical gyre and the Kuroshio Extension (KE) and Oyashio Extension (OE) fronts have moved poleward in the past few decades. However, changes of the North Pacific Subtropical Fronts (STFs), anchored by the North Pacific subtropical countercurrent in the southern subtropical gyre, remain to be quantified. By synthesizing observations, reanalysis, and eddy-resolving ocean hindcasts, we show that the STFs, especially their eastern part, weakened (20% ± 5%) and moved poleward (1.6° ± 0.4°) from 1980 to 2018. Changes of the STFs are modified by mode waters to the north. We find that the central mode water (CMW) (180°–160°W) shows most significant weakening (18% ± 7%) and poleward shifting (2.4° ± 0.9°) trends, while the eastern part of the subtropical mode water (STMW) (160°E–180°) has similar but moderate changes (10% ± 8%; 0.9° ± 0.4°). Trends of the western part of the STMW (140°–160°E) are not evident. The weakening and poleward shifting of mode waters and STFs are enhanced to the east and are mainly associated with changes of the northern deep mixed layers and outcrop lines—which have a growing northward shift as they elongate to the east. The eastern deep mixed layer shows the largest shallowing trend, where the subduction rate also decreases the most. The mixed layer and outcrop line changes are strongly coupled with the northward migration of the North Pacific subtropical gyre and the KE/OE jets as a result of the poleward expanded Hadley cell, indicating that the KE/OE fronts, mode waters, and STFs change as a whole system.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The western North Pacific Subtropical Countercurrent (WSTCC), anchored by the western subtropical front (WSTF), varies from seasonal to multidecadal time scales and affects the climate variability in the North Pacific. However, conflicting views exist for physical mechanisms of the decadal to multidecadal variability of the WSTF/WSTCC. While many previous studies attribute the WSTF/WSTCC decadal variability to the anomalous wind forcing by the Pacific Decadal Oscillation, by synthesizing observations, reanalysis products and the pre‐industrial model simulation experiment, the present study demonstrates that they are controlled by the Atlantic Multidecadal Oscillation (AMO) through the Subtropical Mode Water (STMW) variability. The AMO induced decadal temperature and volume anomalies of the STMW are stored in the subsurface ocean through the subduction process south of the Kuroshio Extension. Then these anomalies propagate southwestward along the thermocline circulation and reaches the northern flank of the WSTCC in about 5 years. During the AMO‐positive phase, the anomalous warm and thin STMW flattens the upper thermocline, reduces the meridional temperature gradient and weakens the WSTF intensity, lagging 5 years behind the AMO index. Subsequently, the WSTCC decreases and moves southward, and vice versa for the AMO‐negative phase. Our results provide a potential predictability for the WSTF and WSTCC decadal to multidecadal variability and are important for the North Pacific climate change and the biogeochemical processes.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The meridional shift of the Kuroshio Extension (KE) front and changes in the formation of the North Pacific Subtropical Mode Water (STMW) during 1979–2018 are reported. The surface-to-subsurface structure of the KE front averaged over 142°–165°E has shifted poleward at a rate of ~0.23° ± 0.16° decade<jats:sup>−1</jats:sup>. The shift was caused mainly by the poleward shift of the downstream KE front (153°–165°E, ~0.41° ± 0.29° decade<jats:sup>−1</jats:sup>) and barely by the upstream KE front (142°–153°E). The long-term shift trend of the KE front showed two distinct behaviors before and after 2002. Before 2002, the surface KE front moved northward with a faster rate than the subsurface. After 2002, the surface KE front showed no obvious trend, but the subsurface KE front continued to move northward. The ventilation zone of the STMW, defined by the area between the 16° and 18°C isotherms or between the 25 and 25.5 kg m<jats:sup>−3</jats:sup> isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth <jats:italic>h</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub> before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%; 1 Sv ≡ = 10<jats:sup>6</jats:sup> m<jats:sup>3</jats:sup> s<jats:sup>−1</jats:sup>) during 1979–2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction (−∂<jats:italic>h</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub>/∂<jats:italic>t</jats:italic>) was dominant accounting for 91% of the rate reduction, while the vertical pumping (−<jats:italic>w</jats:italic><jats:sub>mb</jats:sub>) amounted to 8% and the lateral induction (−<jats:bold>u</jats:bold><jats:sub>mb</jats:sub> ⋅ ∇<jats:italic>h</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub>) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed <jats:italic>h</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub> that were incurred by the poleward shift of KE front.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The decadal to multidecadal mixed layer variability is investigated in a region south of the Kuroshio Extension (130°E–180°, 25°–35°N), an area where the North Pacific subtropical mode water forms, during 1948–2012. By analyzing the mixed layer heat budget with different observational and reanalysis data, here we show that the decadal to multidecadal variability of the mixed layer temperature and mixed layer depth is covaried with the Atlantic multidecadal oscillation (AMO), instead of the Pacific decadal oscillation (PDO). The mixed layer temperature has strong decadal to multidecadal variability, being warm before 1970 and after 1990 (AMO positive phase) and cold during 1970–90 (AMO negative phase), and so does the mixed layer depth. The dominant process for the mixed layer temperature decadal to multidecadal variability is the Ekman advection, which is controlled by the zonal wind changes related to the AMO. The net heat flux into the ocean surface <jats:italic>Q</jats:italic><jats:sub>net</jats:sub> acts as a damping term and it is mainly from the effect of latent heat flux and partially from sensible heat flux. While the wind as well as mixed layer temperature decadal changes related to the PDO are weak in the western Pacific Ocean. Our finding proposes the possible influence of the AMO on the northwestern Pacific Ocean mixed layer variability, and could be a potential predictor for the decadal to multidecadal climate variability in the western Pacific Ocean.</jats:p>